Our question is made explicit by posing it within the family of general
probabilistic theories (GPTs) which have emerged as natural generalizations
of quantum theory [1]}, [2]}, [3]}.
The framework of GPTs derives from operational principles and it encompasses
both quantum and classical models. One of the motivations to explore
these alternative theories has been to identify features which single
out quantum theory among others of comparable structure. Our study
contributes to that fundamental quest.
